Electrical generators are used in a wide variety of applications. Typically, an individual electrical generator operates in a stand-by mode wherein the electrical power provided by a utility is monitored such that if the commercial electrical power from the utility fails, the engine of the electrical generator is automatically started causing the alternator to generate electrical power. When the electrical power generated by the alternator reaches a predetermined voltage and frequency desired by the customer, a transfer switch transfers the load imposed by the customer from the commercial power lines to the electrical generator.
Typically, electrical generators utilize a single driving engine coupled to a generator or alternator through a common shaft. Upon actuation of the engine, the crankshaft rotates the common shaft so as to drive the alternator that, in turn, generates electrical power. As is known, most residential electric equipment in the United States is designed to be used in connection with electrical power having a fixed frequency, namely, sixty (60) hertz (Hz). The frequency of the output power of most prior electrical generators depends on a fixed, operating speed of the engine. Typically, the predetermined operating speed of an engine for a two-pole, stand-by electrical generator is approximately 3600 revolutions per minute to produce the rated frequency and power for which the unit is designed. However, in situations when the applied load is the less than the rated kilowatt load for which the unit is designed, the fuel-efficiency of the engine will be less than optimum. As such, it can be appreciated that it is highly desirable to vary the operating speed of the engine of an electrical generator to maximize fuel efficiency, and thus reduce CO2 emissions, of the engine for a given load. Further, operation of the engine-driven, electrical generator at its predetermined operating speed can produce unwanted noise. It can be appreciated that reducing the operating speed of the engine of an electrical generator to correspond to a given load will reduce the noise associated with operation of the engine-driven, electrical generator.
Therefore, it is a primary object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator.
It is a further object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator that maximize fuel efficiency of the engine for a given load.
It is a still further object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator that is simple and that reduces the overall cost of operation of the generator.
It is a still further object and feature of the present invention to provide a method for controlling a variable speed, constant frequency, stand-by electrical generator that minimizes the noise associated with operation of the generator.
In accordance with the present invention, a method of controlling an engine-driven, electrical generator is provided. The generator generates an output voltage at a frequency with the engine running at an operating speed. The method includes the steps of connecting the generator to a load and varying the operating speed of the engine to optimize fuel consumption in response to the load. Thereafter, the frequency of the output voltage is modified to a predetermined level.
The step of modifying the frequency of the output voltage includes the additional steps of calculating the difference between the frequency of the output voltage and the predetermined level and providing the difference as an adjustment frequency. The frequency of the output voltage is modified by the adjustment frequency. The generator includes a rotor having windings and stator having an output. The output of the stator is connectable to the load. In addition, the output of the stator is operatively connected to an input of an inverter. The inverter receives the output voltage at the frequency. The output of the inverter is operatively connected to the windings of the rotor. The inverter supplies power to the rotor windings at the adjustment frequency. The stator has a main winding and a quadrature winding, and the inverter includes a DC link. The sensing input of the inverter is operatively connected to the main winding and power for the DC link is operatively connected to the quadrature winding. It is contemplated for the predetermined level of the unmodified frequency to be in the range of 40 to 75 hertz and for the engine to have a minimum operating speed of approximately 2400 revolutions per minute.
In accordance with a further aspect of the present invention, a method of controlling an engine-driven, electrical generator including a rotor and a stator having an output is provided. The generator generates an output voltage at a frequency at the stator output with the engine running at an engine speed. The method includes the steps of connecting the output of the stator to a load and adjusting the engine speed in response to the load. The difference between the frequency of the output voltage and a predetermined level is calculated and the difference is provided as an adjustment frequency. The frequency of the output voltage is modified by the adjustment frequency.
The generator includes a rotor having windings and the method includes the additional step of operatively connecting the output of the stator to an input of an inverter. The inverter receives the output voltage at the frequency. An output of the inverter is operatively connected to the windings of the rotor. The inverter supplies power to the rotor windings at the adjustment frequency.
The stator has a main winding and a quadrature winding, and the inverter includes a DC link. The input of the inverter is operatively connected to the main winding and the DC link is operatively connected to the quadrature winding. It is contemplated for the predetermined level of the frequency is in the range of 40 to 75 hertz and for the engine to have a minimum operating speed of approximately 2400 revolutions per minute.
In accordance with a still further aspect of the present invention, a method of controlling an engine-driven, electrical generator including a rotor having rotor windings and stator having an output is provided. The generator generates an output voltage at a frequency at the stator output with the engine running at an engine speed. The method includes the steps of connecting the output of the stator to a load and adjusting the engine speed in response to the load. Slip power is supplied to the rotor windings to adjust the frequency of the output voltage to a predetermined level.
The step of supplying slip power to the rotor windings includes the additional steps of calculating the difference between the frequency of the output voltage and the predetermined level and providing the difference as an adjustment frequency. The slip power has a frequency generally equal to the adjustment frequency. The output of the stator is operatively connected to an input of an inverter. The inverter receives the output voltage at the frequency. An output of the inverter is operatively connected to the windings of the rotor. The inverter supplies the slip power to the rotor windings at the adjustment frequency.
The stator has a main winding and a quadrature winding, and the inverter includes a DC link. The input of the inverter is operatively connected to the main winding and the DC link is operatively connected to the quadrature winding. It is contemplated for the predetermined level of the frequency is in the range of 40 to 75 hertz and for the engine to have a minimum operating speed of approximately 2400 revolutions per minute. The engine speed is adjusted to optimize fuel consumption in response to the load thereon.